1. Field of the Invention
This invention relates generally to operational amplifier circuits, and more particularly, to an auto-zeroing operational amplifier circuit.
2. Description of the Prior Art
In general, operational amplifier offset voltage compensation circuits have reduced offset voltage error by applying a cancelling voltage at the input of the operational amplifier in one of two ways. One way is to utilize the power supply to generate a cancelling voltage of opposite polarity to the offset voltage across a resistor divider network. In general, such circuits required at least one variable resistor to facilitate compensation for the numerous variable circuit parameters which affect the value of the offset voltage. Such circuits have been found to be unsatisfactory in some situations since they require constant adjustment of the variable resistance and tend to introduce noise from the power supply.
A second way to reduce offset voltage error with a cancelling voltage at the operational amplifier's input is to utilize switched capacitor techniques to periodically charge a capacitor, external to the integrated circuit containing the operational amplifier, to a voltage approximately equal but of opposite polarity from the offset voltage. Such switched capacitor circuits are susceptible to external noise due to the high impedance of the external circuit node connection, and the external capacitor also increases the pin count of the integrated circuit.